Packaging2

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* Normalize post-release spellings for rev/r prefixes. `35 <https://github.com/pypa/packaging/issues/35>`__

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* Fix an error where the arbitrary specifier (``===``) was not correctly
allowing pre-releases when it was being used.

* Expose the specifier and version parts through properties on the
``Specifier`` classes.

* Allow iterating over the ``SpecifierSet`` to get access to all of the
``Specifier`` instances.

* Allow testing if a version is contained within a specifier via the ``in``
operator.

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* Fix a logic error that was causing inconsistent answers about whether or not
a pre-release was contained within a ``SpecifierSet`` or not.

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* Add ``Version().is_postrelease`` and ``LegacyVersion().is_postrelease`` to
make it easy to determine if a release is a post release.

* Add ``Version().base_version`` and ``LegacyVersion().base_version`` to make
it easy to get the public version without any pre or post release markers.

* Support the update to PEP 440 which removed the implied ``!=V.*`` when using
either ``>V`` or ``<V`` and which instead special cased the handling of
pre-releases, post-releases, and local versions when using ``>V`` or ``<V``.

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* Normalize release candidates as ``rc`` instead of ``c``.

* Expose the ``VERSION_PATTERN`` constant, a regular expression matching
a valid version.

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* Ensure that versions are normalized before comparison when used in a
specifier with a less than (``<``) or greater than (``>``) operator.